Samples of topsoil (0-30 cm) and dry season herbage from 135 sites in the Mt Elgon region of Kenya were classified according to farm (n = 84), site altitude, underlying soil bedrock (6 types) and botanical composition (6 classes). Effects on pasture concentrations of Ca, P, Mg and S were determined using a mixed model for unbalanced data sets and the Wald (W) statistic to assess the significance of fixed effects. Associated effects on pH, plus extractable Ca and P concentrations in the topsoils were also evaluated.
Soil bedrock influenced herbage concen-trations of S (P < 0.001) but not those of Ca, P or Mg. Mean herbage S concentrations were lowest on volcanic and metamorphic gneiss associations (1.2 g/kg DM) but only extreme values would be inadequate for grazing livestock. Altitude appeared to affect the concentration of P (P < 0.01) and not those of Ca, Mg and S in herbage but the effect on P was dependent on soil P. Geological and topographical maps cannot be used to predict macro-mineral disorders in live¬stock in the Mt Elgon region.
Herbage species differed markedly in their concentrations of S (P < 0.001), Ca (P < 0.001) and Mg (P < 0.05) but not P. Ca deficiency may arise on setaria, S deficiency on some napier grass pastures and P deficiency on some dry season pastures irrespective of botanical compo¬sition.
Low herbage P concentrations may reflect advanced maturity rather than low soil P status (mean value 20 mgP/kg DM). The correlation between soil P and herbage P was significant (r = 0.595), and similar in slope and intercept for all herbage classes but not strong enough to predict deficient Ijerbages. Herbage Ca was not corre¬lated with soil Ca.